The broad objective is to describe and interpret the behavior in an open community population of types A and B influenza viruses and of rhinoviruses. This will be based on careful analysis of household episodes of infection with specific viruses detected and observed through intensive continuing surveillance of families with children for illness and for infection as revealed by virus isolation from frequent routinely scheduled and special illness specimens and by serology. Questions relating to both virus groups concern: viral persistence in the population; usual paths of community spread; effectiveness of homotypic immunity as revealed by infection, disease and virus shedding following household exposure; relation of household spread of infection and proportion of infections resulting in disease to virus serotype, virus strains and to the clinical or subclinical nature of the introducing infection; and the relation of homotypic antibody to response to virus serotype, occurrence of associated disease and prior antibody level. For influenzaviruses, special questions include: the importance of anti-nueraminidase antibody; pattern of prevalence of protective antibodies as basis for prediction of epidemics; correlation between antigenic drift and the occurrence of reinfection; and the significance of reinfection in virus spread and as cause of disease. For rhinoviruses, key questions relate to: the existence and identity of "common" serotypes; host (familial) vulnerability to disease; and variation in pathogenicity between and within serotypes.